My 99 GT is doing the same thing. It will shift smooth until the temp gets about 175-185 on the gauge then it will start jumping. I guess its getting worse now because it only use to start jumping at 210 which is normal temp. However I do have a heavy foot and tend to drive aggressively. But not on hot days where it would start jumping. Usually at night in cooler temps. Although I race my car I also maintain it very well with tune ups and fluid changes. My most current issue is now my transmission is slipping at wot from 2-3 but if I drive normal it goes in every gear smoothly until the tranny heats up :P
